1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device including a polarizer of which a retardation value is variable on regions for compensation of gravity defect caused by an excess liquid crystal at room temperature.
2. Discussion of the Related Art
Demands for various display devices have increased with development of an information society. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some species of flat display devices have already been applied to displays for various equipments.
Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to their advantageous characteristics of a thin profile, light weight, and low power consumption, whereby the LCD devices provide a substitute for a Cathode Ray Tube (CRT). In addition to mobile type LCD devices such as a display for a notebook computer, LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology having applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various environments as a general purpose display, the key to developing LCD devices depends on whether LCD devices can implement a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
Hereinafter, a related art LCD device and a problem of gravity defect therein will be described with reference to the accompanying drawings.
FIG. 1 is an exploded perspective view illustrating an LCD device according to the related art. As illustrated in FIG. 1, a related art LCD device includes first and second substrates 1 and 2 bonded to each other at a predetermined interval, and a liquid crystal layer 3 formed between the first and second substrates 1 and 2.
In more detail, the first substrate 1 includes a plurality of gate and data lines 4 and 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the first substrate 1 at fixed intervals, and the plurality of data lines 5 are formed in perpendicular to the plurality of gate lines 4 at fixed intervals. Then, the plurality of pixel electrodes 6, which are arranged in a matrix-type configuration, are respectively formed in pixel regions defined by the plurality of gate and data lines crossing each other. The plurality of thin film transistors T are switched according to signals of the gate lines for transmitting signals of the data lines to the respective pixel electrodes.
The second substrate 2 includes a black matrix layer 7 that excludes light from regions except the pixel regions P of the first substrate, R/G/B color filter layer 8 displaying various colors in correspondence with the pixel regions, and a common electrode 9 to obtain the picture image.
Then, the liquid crystal layer 3 is formed between the first and second substrates 1 and 2. The LCD device is driven according to an electric field generated between the pixel electrode 6 and the common electrode 9. Thus, it is possible to control the amount of light passing through the liquid crystal layer 3 according to the alignment of liquid crystal layer 3, thereby displaying the images.
This is referred to as a TN (Twisted Nematic) mode, which has the disadvantage of a narrow viewing angle. In order to overcome the disadvantage of the TN mode LCD device, an IPS (In-Plane Switching) mode LCD device has been developed and studied.
In case of the IPS mode LCD device, both pixel and common electrodes are formed at a predetermined interval in a pixel region of a first substrate, thereby generating an IPS mode electric field (transverse electric field). That is, a liquid crystal layer is driven with the transverse electric field.
In the meantime, a method for fabricating an LCD device is classified into a liquid crystal dropping method and a liquid crystal injection method according to a type of forming a liquid crystal layer between first and second substrates.
The liquid crystal injection method will be described in brief.
First, a container of receiving a liquid crystal material and an LCD panel are positioned inside a chamber. According as the chamber is maintained under a predetermined pressure, moisture and foam is removed from the liquid crystal material and the inner walls of the container, and the inner space of the LCD panel is made to a vacuum state.
A liquid crystal inlet of the LCD panel is dipped into the liquid crystal material of the container under the vacuum state, and then the pressure inside the chamber is made to an atmospheric pressure. Thus, the liquid crystal material is injected to the inside of the LCD panel through the liquid crystal inlet by the pressure difference between the inside of the LCD panel and the chamber.
A method for fabricating an LCD device by the liquid crystal injection method has the following disadvantages. After cutting the two substrates into unit LCD panels, the vacuum state is maintained between the two substrates. In this state, the liquid crystal inlet is dipped into the container of the liquid crystal material. Thus, it requires a long time to inject liquid crystal, thereby lowering the manufacturing yield.
Also, when forming the large-sized LCD device, it is impossible to completely inject the liquid crystal material to the inside of the LCD panel, thereby causing the defects.
Further, the liquid crystal injection method requires a complicated process and spends a long time. That is, it is necessary to provide several equipments for injection of liquid crystal, thereby requiring a large space.
To overcome these problems of the liquid crystal injection method, the liquid crystal dropping method is developed, in which liquid crystal is dropped on any one of two substrates, and the two substrates are bonded to each other. In the liquid crystal dropping method, before bonding the two substrates to each other, the appropriate amount of liquid crystal is dropped on any one of the two substrates. After completing the dropping of liquid crystal, the two substrates are bonded to each other.
FIG. 2 is a photograph illustrating a gravity defect in an LCD device according to the related art. FIG. 3 is a cross sectional view along I-I′ of FIG. 2.
Referring to FIG. 2, the gravity defect generates in a lower edge of the LCD panel which is adjacent to the ground. When the LCD panel stands vertically, liquid crystal flows downward by gravity. Thus, the lower edge of the LCD panel is swollen, whereby a cell gap corresponding to the lower edge of the LCD panel increases, thereby generating spots.
As illustrated in FIG. 3, when cutting the LCD panel 10, a column spacer 20 may be apart from the first substrate 1 due to the swollen liquid crystal in the lower edge of the LCD panel 10. Especially, if the LCD panel 10 stands vertically when providing the excessive amount of liquid crystal, the liquid crystal flows to the lower edge of the LCD panel which is adjacent to the ground, whereby the lower edge of the LCD panel is relatively swollen.
At this time, a sealant pattern 25 is formed in a peripheral region of the LCD panel 10, and then the first and second substrates 1 and 2 are bonded to each other by the sealant pattern 25. In this case, a liquid crystal layer 3 is formed with the liquid crystal dropped between the first and second substrates 1 and 2. In the liquid crystal layer 3, a cell gap in the portion having no gravity defect is different from a cell gap in the portion having the gravity defect. When driving the LCD device, the portion having the gravity defect is displayed as a blurred image.
FIG. 4 is a view illustrating a polarizer corresponding to the LCD panel according to the related art. Referring to FIG. 4, the polarizer 30 corresponding to the LCD panel has the same retardation value Δnd on the entire surface.
When the LCD panel stands vertically, the liquid crystal flows down. Thus, the lower edge of the LCD panel swells, which is displayed as the blurred portion, thereby deteriorating the picture quality. The gravity defect is generated when the liquid crystal is excessively dropped on the substrate. Even though the LCD panel is maintained at room temperature or high temperature, the gravity defect may nonetheless be generated.
Thus, as an example, the related art LCD device has the disadvantage of when the LCD panel stands vertically after excessive dropping of the liquid crystal on the substrate, the liquid crystal flows downward, whereby the lower edge of the LCD panel becomes swollen, which is referred to as the gravity defect. In this case, the LCD panel having the gravity defect may result in a blurred image, lowering the picture quality.